Ultraviolet-curable coatings based on acrylate-terminated polyurethanes are well known and in use to provide rapidly curing coatings for the coating of optical glass fiber. In some instances the optical glass fiber is coated with a primary coating comprising such acrylate-terminated polyurethanes and then overcoated. In other instances the optical glass fiber is coated with a single coating comprising such acrylate-terminated polyurethanes. In either instance it is essential to provide ultraviolet-curing coatings which cure rapidly on exposure to such radiation and which provide good water resistance and improved adhesion to the glass surface.
Acrylated polyurethanes based on polyether diols are frequently used in optical fiber coatings, but the polyether linkages in these diols do not provide the best water resistance. To improve the water resistance, efforts have been made to use diols or other polyhydric alcohols which are polyesters or polycarbonates in the formation of the acrylated polyurethane. The acrylated polyurethanes made with these polyesters or polycarbonates possess better water resistance. However, the low modulus needed for the primary coating or single coating of optical glass fiber requires the addition of greater amounts of reactive diluents than when polyethers polyurethanes are used in order to lower the modulus of the coatings so that these polyester or polycarbonate-based polyurethanes can be used. These reactive diluents may contribute to poor adhesion and/or poor water resistance, so this minimizes the utility of the more water-resistant polyurethanes.
It is noted that other agents can be used to enhance the adhesion of ultraviolet curing coatings containing acrylate-terminated polyurethanes, but these other aagents tend to considerably impair the speed of cure. Ultraviolet-curing coatings are used to maximize production speed, and those who use such coatings in the production of coated optical glass fiber are generally unwilling to sacrifice cure speed.
The present invention employs acrylate-terminated polyurethanes in low modulus coatings which exhibit improved water resistance and improved adhesion. It is a feature of this invention to do this using polycarbonate or polyester-based polyurethanes to maximize the water resistance which is obtained, but the conventionally used polyether polyurethanes are also fully useful herein.